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In this case, composition allows the {{CompositeCollection}} class to use its own intrinsic lock in a way that is completely independent of the lock of the underlying list class.  This allows the underlying collection to be not thread-safe because the {{CompositeCollection}} wrapper prevents direct access to its methods by publishing its own synchronized equivalents. This approach provides consistent locking even if the underlying class changes its locking policy in the future. \[[Goetz 06|AA. Java References#Goetz 06]\].